Reoviruses are double-stranded RNA viruses that lack a membrane envelope. Their capsid is organised in two concentric icosahedral layers: an inner core and an outer capsid layer. The sigma1 protein is found in the outer capsid, and the sigma2 protein is found in the core. There are four other kinds of protein (besides sigma2) in the core, termed lambda 1-3, mu2. Interactions between sigma2 and lambda 1 and lambda 3 are thought
to initiate core formation, followed by mu2 and lambda2 [9971813].

Sigma1 is a trimeric protein, and is positioned at the 12 vertices of the icosahedral outer capsid layer. Its N-terminal fibrous tail, arranged as a triple coiled coil,
anchors it in the virion, and a C-terminal globular head interacts with the
cellular receptor [11438552]. These two parts form by separate trimerization events.
The N-terminal fibrous tail forms on the polysome, without the involvement
of ATP or chaperones. The post- translational assembly of the C-terminal
globular head involves the chaperone activity of Hsp90, which is associated
with phosphorylation of Hsp90 during the process [11438552]. Sigma1 protein acts
as a cell attachment protein, and determines viral virulence, pathways of
spread, and tropism. Junctional adhesion molecule has been identified as a
receptor for sigma1 [11239401]. In type 3 reoviruses, a small region, predicted to
form a beta sheet, in the N-terminal tail was found to bind target cell surface
sialic acid (i.e. sialic acid acts as a co-receptor) and promote apoptosis [11287552].
The sigma1 protein also binds to the lambda2 core protein [9311901].

Browse genome assignments for this superfamily. The SUPERFAMILY hidden Markov model library has been used to carry
out SCOP domain assignments to all genomes at the superfamily level.

Alignments of sequences to 1 models
in this superfamily are available by clicking on the 'Alignments' icon above. PDB sequences less than 40% identical
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Browse and view proteins in genomes which have
different domain combinations including a Outer capsid protein sigma 3 domain.

Examine the distribution of domain superfamilies, or families, across the major taxonomic kingdoms or genomes within a kingdom. This gives an immediate impression of how superfamilies, or families, are restricted to certain kingdoms of life.

There are 1 hidden Markov models representing the Outer capsid protein sigma 3 superfamily. Information on how the models are built, and plots showing hydrophobicity, match emmission probabilities and insertion/deletion probabilities can be inspected.